1. Field of the Invention
The present invention relates to a developer carrying member used in a developing apparatus to form a toner image by developing an electrostatic latent image formed on the image carrying member such as an electrophotographic photosensitive member or electrostatic recording dielectric. The invention also relates to a developing method using the above described developer carrying member.
2. Related Background Art
Electrophotography conventionally forms an electrostatic latent image on the support thereof (photosensitive drum) by various measures using a photoconductive material, then develops the electrostatic latent image by a developer (toner) to form the toner image and transfer the toner image on a transfer material, such as paper, if appropriate, followed by fixation of the toner image on the transfer material by application of heat, pressure or both heat and pressure to obtain the print or copy.
Developing systems in the electrophotography are principally classified into the one-component and two-component developing systems. Recently, since there are needs of miniaturization of the developing apparatus part aiming at a lightweight and miniaturized electrophotographic apparatus, the one-component developing system is often used.
Since the one-component developing system does not require the carrier particles as the two-component developing system does, the developing apparatus itself can be miniaturized and light. On the other hand, the two-component developing system requires the constant toner density to be kept in the developer, therefore needed is the apparatus for detecting the toner density and supplying the needed amount of toner, accordingly, the developing apparatus will be larger and heavier. Since the one-component developing system does not need such an apparatus, it can be smaller and lighter.
In the developing apparatus using the one-component developing system, the electrostatic latent image is formed on the surface of the photosensitive drum as a carrying member of the electrostatic latent image, and provide the toner with positive or negative friction charge through friction between the developer carrying member carrying member (developing sleeve) and toner and/or friction between the member regulating the thickness of the developer layer and toner. Then the toner electrified is applied thin on the developing sleeve which is conveyed to the developing region where the photosensitive drum is opposed to the developing sleeve. In the developing region, the toner is attached to the electrostatic latent image on the surface of the photosensitive drum to develop and form the toner image.
When using such a one-component developing system, homogenized toner charge and sufficient endurance stability are needed.
Particularly, the charge-up phenomenon is likely to occur particularly under low humidity where the charge amount of the toner coated on the developing sleeve becomes excessively high owing to the contact with the developing sleeve during repeated rotation of the developing sleeve, resulting in immobilization of the toner on the developing sleeve by drawing between the toner and the reflection force on the developing sleeve failing in transfer of the toner from the developing sleeve to the electrostatic latent image on the photosensitive drum that is charge-up phenomenon. When the charge-up phenomenon occurs, it becomes difficult for the toner in the upper layer to charge resulting in reduction of developing amount of the toner. Consequently, there sometimes occur problems such as thinning of the line image and lowering of image density of the solid image. Further, the toner which has failed in appropriate charging owing to the charge-up phenomenon may flow on the developing sleeve off the control to make spotty or wavy unevenness that is the blotching phenomenon.
Furthermore, a sleeve ghost phenomenon, indicating a visible trace of solid image likely to occur on the image when the position where the solid image once developed with high image density on the developing sleeve comes to the developing position at the following rotation of the developing sleeve to develop the half-tone image.
Recently, reduction of the particles size and fine-granulation of the toner have been attempted for digitization of the electrophotography apparatus and for higher image quality. For example, the toner which has about 5 to 12 μm of the weight average particles size is used in general in order to enhance the resolution and sharpness of letters reproducing the constant electrostatic latent image.
Further, in view of saving energy and space of office, miniaturization of the printer is required. Consequently, miniaturization of the container storing the toner in the printer is also required and the low consumption toner which enables printing out a large number of sheets by small amount of the toner should be used. As a low consumption toner, the toner wherein the form of the toner particles is approximated spherical has been used.
Furthermore, the tendency is decrease of a fixation temperature for the purpose of time reduction for fast copying and saving electric power.
In such situations, the toner, particularly under a low temperature and low humidity is more likely to attach electrostatistically on the developing sleeve because of increased charge per unit weight, while under high temperature and high humidity, blotching and melt-adhesion by the toner are likely to occur on the developing sleeve.
As a method to solve such phenomena, in publication of Japanese Patent Application Laid-Open No. 1-276174, proposed is using in the developing apparatus a developing sleeve wherein a resin-coated layer with an electroconductive fine powder such as crystalline graphite or carbon dispersed in the resin is set on a metal substrate. By using this developing sleeve, substantial reduction of the above phenomena is noted.
In this developing sleeve, however, when adding much amount of electroconductive fine powder, appropriate electrification to the toner is decreased leading to difficulty of obtaining high image density particularly in the environment of high temperature and high humidity, though the case is good for charge-up and sleeve ghost. Further, when adding much amount of electroconductive fine powder, the resin-coated layer becomes friable being easy to be scraped as well as configuration of the surface is likely to be uneven, and when advancing endurance for a large number of sheets, surface roughness and surface composition of the resin-coated layer is altered resulting in frequent occurrence of poor conveyance of the toner and inhomogeneous electrification to the toner.
In publication of Japanese Patent Application Laid-Open No. 1-276174, proposed is a developing apparatus having a developing sleeve which uses a coated layer with crystalline graphite particles dispersed. The crystalline graphite particles used there are those comprised of artificial graphite, which is obtained by burning a shaped aggregate, such as coke bound by tar pitch at about 1,000 to 1,300° C., and then graphitizing it at about 2,500 to 3,000° C., or natural graphite. Accordingly, the crystalline graphite has lubricity caused by the scaly structure which exerts effect against charge-up and sleeve ghost. However, the crystalline graphite particles are scaly and indeterminate in shape, and in addition, when they are dispersed in the resin-coated layer, it is difficult for the particles to be smaller and dispersed evenly, resulting in an uneven surface of the resin-coated layer. Such an uneven surface formed by the crystalline graphite may cause melt-adhesion of the toner thereto.
Further, owing to the low hardness of the above crystalline graphite, abrasion and elimination of the crystalline graphite particles themselves are likely to occur on the surface of the resin-coated layer. Accordingly, the surface roughness and surface composition of the resin-coated layer are likely to change when advancing endurance for a large number of sheets which leads to frequent occurrence of the toner melt-adhesion, consequently, poor conveyance of the toner and inhomogeneous electrification to the toner are likely to occur. On the other hand, when adding small amount of electroconductive fine powder such as carbon to the resin-coated layer formed on the metal substrate of the developing sleeve, the effect of the crystalline graphite particles and electroconductive fine powder is weak, accordingly, charge-up and sleeve ghost may occur.
In publication of Japanese Patent Application Laid-Open No. 3-200986, proposed is a developing apparatus having a developing sleeve wherein on the metal substrate, electrically conductive resin-coated layer is set with electroconductive fine powder such as crystalline graphite and carbon dispersed in the resin. In this developing sleeve, abrasion resistance of the resin-coated layer is improved as well as the surface of the resin-coated layer is made more even, leading to a relatively little change in surface roughness caused by a large number of sheet transfers, which in turn stabilize more the state of the toner coated on the developing sleeve and makes the charge of the toner more uniform. Consequently, problems including sleeve ghost, image density and unevenness of the image density are reduced and the image quality tends to be steadier. Even in this developing sleeve, however, a rapid control for homogeneous charge and stabilization of appropriate electrification to the toner should be preferably improved further more. In addition, for abrasion resistance, change of surface roughness and unevenness of roughness of the resin-coated layer caused by abrasion and elimination of spherical particles or crystalline graphite of the resin in the developing sleeve during use of longer period as well as accompanying toner blotting and toner melt-adhesion of the resin-coated layer are likely to occur. These cases make toner charge unstable often causing poor image including reduction of image density, unevenness of density, fogging and image streaks.
In publication of Japanese Patent Application Laid-Open No. 8-240981, proposed is a developing apparatus having the developing sleeve wherein homogeneous electrification to the toner is improved by homogenizing abrasion resistance and conductivity of the surface of the resin-coated layer caused by homogeneous dispersion of electroconductive spherical particles in the electroconductive resin-coated layer owing to that the spherical particles dispersed in the electroconductive resin-coated layer have lower specific gravity and electroconductivity as well as toner blotting and toner melt-adhesion can be controlled even when the resin-coated layer is worn down to some degree. In this developing sleeve, however, there are matters to be improved regarding rapid and homogeneous electrification to the toner and appropriate electrification to the toner. Further, for endurable use for long time, electroconductive particles such as crystalline graphite are likely to be worn down or eliminated because configuration of the part on the surface of the resin-coated layer without the electroconductive spherical particles is uneven as well as abrasion resistance of the above described part is poor. From such parts which have been worn down and eliminated or from parts of uneven configuration, abrasion of the resin-coated layer and toner blotting as well as toner melt-adhesion occurs which often leads to unstable charge of the toner.
In publication of Japanese Patent Application Laid-Open No. 3-84558, publication of Japanese Patent Application Laid-Open No. 3-229268, publication of Japanese Patent Application Laid-Open No. 4-1766 and publication of Japanese Patent Application Laid-Open No. 4-102862, proposed is a toner in spherical form or the form approximated to the spherical. The developing sleeve and developing apparatus effective for reduction of consumption of the toner and stabilization of development of the toner through endurance has been awaited.
In publication of Japanese Patent Application Laid-Open No. 2-87157, publication of Japanese Patent Application Laid-Open No. 10-97095, publication of Japanese Patent Application Laid-Open No. 11-149176 and publication of Japanese Patent Application Laid-Open No. 11-202557, proposed is a toner which the toner particle shape and surface properties are modified by thermal or mechanical impact of the toner particles synthesized by pulverization method. The developing sleeve and developing apparatus effective for reduction of consumption of the toner and stabilization of development of the toner through endurance has been awaited.